Field of the Invention
The present invention relates to the taking of core samples in subterranean formations, and specifically to the taking of core samples in a direction departing from the sidewall of a borehole.
State of the Art
For many years, geologists in the oil and gas exploration industry have taken and analyzed core samples of potential hydrocarbon-producing formations as part of their efforts to determine the profitability of completing wells from which the core samples are taken, as well as the desirability of further exploratory drilling in the same area. The problem with coring in exploratory wells is the lack of knowledge of the exact location of the formation of interest, or "pay zone," with respect to the well depth. In many instances, drillers have unintentionally drilled completely through the pay zone or target formation without taking any core samples because the pay zone was at a slightly lesser depth than anticipated. Since logging tools can be run into a borehole before it is cased and cemented to determine the location or locations of potential pay zones, coring of these promising zones subsequent to drilling and identification thereof via open-hole logging of the borehole provides a means to verify and enhance the information on production potential provided by seismic surveys and well logs. For obvious reasons, the only economic way to take core samples from a drilled borehole is from its sidewall.
Prior art sidewall coring tools have taken various forms. The most widely used of such tools shoot or punch sample cups into the sidewall of the borehole perpendicular to the borehole axis, and retrieve the cups when the tool is retrieved. Alternatively, tools have been employed which drill cores perpendicular to the borehole. Both types of tools are limited to extremely small diameter, very short cores which do not provide a substantial amount of formation to analyze, and which may therefore not be representative of the target formation characteristics. In addition, such prior art tools often cause damage to the formation by their operation.
Further prior art attempts at sidewall coring using a different approach employed a fixed whipstock within a coring tool run at the end of a drill string. A small core barrel was deployed within the tool adjacent to the whipstock and having a small Moineau-type mud motor and thrusting piston slip joint above the core barrel. At the desired location, an open hole packer on the coring assembly would be inflated by pumping mud down the drill string, the mud flow also powering the motor and thrusting the core barrel down the whipstock and out of the tool at a small angle into the formation. This coring tool also suffered the disadvantages of a small diameter, short (maximum 12") core, and the further limitation that the core sample was taken only at a very slight angle to the borehole, since the whipstock and coring assembly were both carried inside the tool.
Other coring devices which appear to provide the ability for taking core samples at greater angles to the borehole than the above-described internal whipstock tool are disclosed in U.S. Pat. Nos. 2,494,932; 2,558,227; and 2,707,617. These tools, however, employ a small diameter, short coring tool which is rotated via rotation of the drill string transmitted through clutch assemblies. The '617 patent also discloses a variation of the aforementioned punch type of core sampler.
U.S. Pat. No. 4,665,995 discloses an assembly for coring at the bottom of a borehole, a wedge or whipstock being deployed and oriented, and a pilot drilling assembly run down the whipstock at an angle to the borehole. To realm the angled pilot borehole to full diameter, the pilot drilling assembly is withdrawn from the well, and a full bore diameter core barrel is used to ream the pilot hole, following a retrievable pilot spear which is run into the pilot hole to guide the larger core barrel thereinto. In another embodiment, a wedge or whipstock is run into the borehole on a running assembly to a selected location, oriented, and an anchor associated with the whipstock is hydraulically set, after which the running assembly is withdrawn from the well. A full-diameter, standard core barrel is then run into the well and a branch borehole drilled in the direction dictated by the whipstock. The tools and method of the '995 patent, while obviously improvements over the other prior art described herein, suffer the disadvantages of requiring multiple runs or "trips" into and out of the well in order to perform a core sampling operation which results in a full-size core. Moreover, the apparatus of the '995 patent does not provide for retrievability of the whipstock assembly apart from the pilot drilling assembly, thus necessitating the subsequent use of a pilot spear to guide the full-size drilling assembly into the branching off of the main borehole.